diff --git a/icu4c/source/common/filteredbrk.cpp b/icu4c/source/common/filteredbrk.cpp
index 29b02dd38424..a7ac66e7a2d4 100644
--- a/icu4c/source/common/filteredbrk.cpp
+++ b/icu4c/source/common/filteredbrk.cpp
@@ -603,7 +603,11 @@ SimpleFilteredBreakIteratorBuilder::build(BreakIterator* adoptBreakIterator, UEr
   LocalArray<UnicodeString> ustrs(ustrs_ptr);
 
   LocalMemory<int> partials;
-  partials.allocateInsteadAndReset(subCount);
+  // Check subCount > 0 to distinguish allocation failure from empty set (both return nullptr)
+  if (subCount > 0 && partials.allocateInsteadAndReset(subCount) == nullptr) {
+    status = U_MEMORY_ALLOCATION_ERROR;
+    return nullptr;
+  }
 
   LocalPointer<UCharsTrie>    backwardsTrie; //  i.e. ".srM" for Mrs.
   LocalPointer<UCharsTrie>    forwardsPartialTrie; //  Has ".a" for "a.M."
diff --git a/icu4c/source/common/loclikelysubtags.cpp b/icu4c/source/common/loclikelysubtags.cpp
index b37aaeec718e..fb2e43f9d91d 100644
--- a/icu4c/source/common/loclikelysubtags.cpp
+++ b/icu4c/source/common/loclikelysubtags.cpp
@@ -534,7 +534,10 @@ LSR LikelySubtags::makeMaximizedLsrFrom(const Locale &locale,
     }
     LSR max = makeMaximizedLsr(locale.getLanguage(), locale.getScript(), locale.getCountry(),
                             locale.getVariant(), returnInputIfUnmatch, errorCode);
-
+    if (!U_SUCCESS(errorCode)) {
+        return LSR(name, "", "", LSR::EXPLICIT_LSR);
+    }
+    
     if (uprv_strlen(max.language) == 0 &&
         uprv_strlen(max.script) == 0 &&
         uprv_strlen(max.region) == 0) {
diff --git a/icu4c/source/common/normalizer2impl.cpp b/icu4c/source/common/normalizer2impl.cpp
index eb8ed31553c4..99882e6098f1 100644
--- a/icu4c/source/common/normalizer2impl.cpp
+++ b/icu4c/source/common/normalizer2impl.cpp
@@ -533,6 +533,10 @@ Normalizer2Impl::copyLowPrefixFromNulTerminated(const char16_t *src,
     // data and check the first part of the string.
     // After this prefix, determine the string length to simplify the rest
     // of the code.
+    if(src==nullptr) {
+        errorCode=U_ILLEGAL_ARGUMENT_ERROR;
+        return src;
+    }
     const char16_t *prevSrc=src;
     char16_t c;
     while((c=*src++)<minNeedDataCP && c!=0) {}
@@ -1731,6 +1735,9 @@ Normalizer2Impl::composeQuickCheck(const char16_t *src, const char16_t *limit,
     if(limit==nullptr) {
         UErrorCode errorCode=U_ZERO_ERROR;
         src=copyLowPrefixFromNulTerminated(src, minNoMaybeCP, nullptr, errorCode);
+        if (U_FAILURE(errorCode)) {
+            return src;
+        }
         limit=u_strchr(src, 0);
         if (prevBoundary != src) {
             if (hasCompBoundaryAfter(*(src-1), onlyContiguous)) {
diff --git a/icu4c/source/common/ubidiln.cpp b/icu4c/source/common/ubidiln.cpp
index 65e1212c7b26..5e645ede7020 100644
--- a/icu4c/source/common/ubidiln.cpp
+++ b/icu4c/source/common/ubidiln.cpp
@@ -364,6 +364,12 @@ ubidi_getVisualRun(UBiDi *pBiDi, int32_t runIndex,
 {
     int32_t start;
     UErrorCode errorCode = U_ZERO_ERROR;
+    if(pLogicalStart!=nullptr) {
+        *pLogicalStart=0;
+    }
+    if(pLength!=nullptr) {
+        *pLength=0;
+    }
     RETURN_IF_NOT_VALID_PARA_OR_LINE(pBiDi, errorCode, UBIDI_LTR);
     ubidi_getRuns(pBiDi, &errorCode);
     if(U_FAILURE(errorCode)) {
@@ -585,6 +591,10 @@ ubidi_getRuns(UBiDi *pBiDi, UErrorCode*) {
             }
         }
 
+        if (runCount==0) {
+            return false;
+        }
+  
         /*
          * We don't need to see if the last run can be merged with a trailing
          * WS run because setTrailingWSStart() would have done that.
@@ -645,6 +655,7 @@ ubidi_getRuns(UBiDi *pBiDi, UErrorCode*) {
                 /* there is a separate WS run */
                 runs[runIndex].logicalStart=limit;
                 runs[runIndex].visualLimit=length-limit;
+                runs[runIndex].insertRemove=0;
                 /* For the trailing WS run, pBiDi->paraLevel is ok even
                    if contextual multiple paragraphs.                   */
                 if(pBiDi->paraLevel<minLevel) {
diff --git a/icu4c/source/common/ubidiwrt.cpp b/icu4c/source/common/ubidiwrt.cpp
index fb5adb2e5e3c..38b86f9995c1 100644
--- a/icu4c/source/common/ubidiwrt.cpp
+++ b/icu4c/source/common/ubidiwrt.cpp
@@ -60,16 +60,12 @@ doWriteForward(const char16_t *src, int32_t srcLength,
                char16_t *dest, int32_t destSize,
                uint16_t options,
                UErrorCode *pErrorCode) {
-    if (srcLength<=0) {
-        *pErrorCode=U_ILLEGAL_ARGUMENT_ERROR;
-        return 0;
-    }
     /* optimize for several combinations of options */
     switch(options&(UBIDI_REMOVE_BIDI_CONTROLS|UBIDI_DO_MIRRORING)) {
     case 0: {
         /* simply copy the LTR run to the destination */
         int32_t length=srcLength;
-        if(destSize<length) {
+        if(!destSize || destSize<length) {
             *pErrorCode=U_BUFFER_OVERFLOW_ERROR;
             return srcLength;
         }
@@ -83,7 +79,7 @@ doWriteForward(const char16_t *src, int32_t srcLength,
         int32_t i=0, j=0;
         UChar32 c;
 
-        if(destSize<srcLength) {
+        if(!destSize || destSize<srcLength) {
             *pErrorCode=U_BUFFER_OVERFLOW_ERROR;
             return srcLength;
         }
@@ -178,11 +174,6 @@ doWriteReverse(const char16_t *src, int32_t srcLength,
     int32_t i, j;
     UChar32 c;
 
-    if(srcLength<=0) {
-        *pErrorCode=U_ILLEGAL_ARGUMENT_ERROR;
-        return 0;
-    }
-
     /* optimize for several combinations of options */
     switch(options&(UBIDI_REMOVE_BIDI_CONTROLS|UBIDI_DO_MIRRORING|UBIDI_KEEP_BASE_COMBINING)) {
     case 0:
@@ -192,7 +183,7 @@ doWriteReverse(const char16_t *src, int32_t srcLength,
          * and there is no mirroring and no keeping combining characters
          * with their base characters.
          */
-        if(destSize<srcLength) {
+        if(!destSize || destSize<srcLength) {
             *pErrorCode=U_BUFFER_OVERFLOW_ERROR;
             return srcLength;
         }
@@ -220,7 +211,7 @@ doWriteReverse(const char16_t *src, int32_t srcLength,
          * and there is no mirroring.
          * We do need to keep combining characters with their base characters.
          */
-        if(destSize<srcLength) {
+        if(!destSize || destSize<srcLength) {
             *pErrorCode=U_BUFFER_OVERFLOW_ERROR;
             return srcLength;
         }
@@ -269,7 +260,7 @@ doWriteReverse(const char16_t *src, int32_t srcLength,
             src-=srcLength;
         }
 
-        if(destSize<i) {
+        if(!destSize || destSize<i) {
             *pErrorCode=U_BUFFER_OVERFLOW_ERROR;
             return i;
         }
diff --git a/icu4c/source/common/ucnv.cpp b/icu4c/source/common/ucnv.cpp
index c6c5084a7840..fd1c1177346e 100644
--- a/icu4c/source/common/ucnv.cpp
+++ b/icu4c/source/common/ucnv.cpp
@@ -843,6 +843,7 @@ _fromUnicodeWithCallback(UConverterFromUnicodeArgs *pArgs, UErrorCode *err) {
     const char16_t *realSource, *realSourceLimit;
     int32_t realSourceIndex;
     UBool realFlush;
+    UBool needRestore=false;
 
     cnv=pArgs->converter;
     s=pArgs->source;
@@ -891,6 +892,7 @@ _fromUnicodeWithCallback(UConverterFromUnicodeArgs *pArgs, UErrorCode *err) {
         pArgs->sourceLimit=replay-cnv->preFromULength;
         pArgs->flush=false;
         sourceIndex=-1;
+        needRestore=true;
 
         cnv->preFromULength=0;
     }
@@ -982,6 +984,7 @@ _fromUnicodeWithCallback(UConverterFromUnicodeArgs *pArgs, UErrorCode *err) {
                     if((sourceIndex+=cnv->preFromULength)<0) {
                         sourceIndex=-1;
                     }
+                    needRestore=true;
 
                     cnv->preFromULength=0;
                 } else {
@@ -1002,7 +1005,7 @@ _fromUnicodeWithCallback(UConverterFromUnicodeArgs *pArgs, UErrorCode *err) {
                      * (continue converting by breaking out of only the inner loop)
                      */
                     break;
-                } else if(realSource!=nullptr) {
+                } else if(needRestore) {
                     /* switch back from replaying to the real source and continue */
                     pArgs->source=realSource;
                     pArgs->sourceLimit=realSourceLimit;
@@ -1010,6 +1013,7 @@ _fromUnicodeWithCallback(UConverterFromUnicodeArgs *pArgs, UErrorCode *err) {
                     sourceIndex=realSourceIndex;
 
                     realSource=nullptr;
+                    needRestore=false;
                     break;
                 } else if(pArgs->flush && cnv->fromUChar32!=0) {
                     /*
@@ -1065,7 +1069,7 @@ _fromUnicodeWithCallback(UConverterFromUnicodeArgs *pArgs, UErrorCode *err) {
                      * copied back into the UConverter
                      * and the real arguments must be restored
                      */
-                    if(realSource!=nullptr) {
+                    if(needRestore) {
                         int32_t length;
 
                         U_ASSERT(cnv->preFromULength==0);
@@ -1079,6 +1083,7 @@ _fromUnicodeWithCallback(UConverterFromUnicodeArgs *pArgs, UErrorCode *err) {
                         pArgs->source=realSource;
                         pArgs->sourceLimit=realSourceLimit;
                         pArgs->flush=realFlush;
+                        needRestore=false;
                     }
 
                     return;
diff --git a/icu4c/source/common/ucnv2022.cpp b/icu4c/source/common/ucnv2022.cpp
index df6f0cb9ba6d..34e4bf4bbc77 100644
--- a/icu4c/source/common/ucnv2022.cpp
+++ b/icu4c/source/common/ucnv2022.cpp
@@ -178,13 +178,18 @@ enum { MAX_JA_VERSION=0 };
 #else
 enum { MAX_JA_VERSION=4 };
 #endif
-static const uint16_t jpCharsetMasks[MAX_JA_VERSION+1]={
+static const uint16_t jpCharsetMasks[5]={
     CSM(ASCII)|CSM(JISX201)|CSM(JISX208)|CSM(HWKANA_7BIT),
 #if !UCONFIG_ONLY_HTML_CONVERSION
     CSM(ASCII)|CSM(JISX201)|CSM(JISX208)|CSM(HWKANA_7BIT)|CSM(JISX212),
     CSM(ASCII)|CSM(JISX201)|CSM(JISX208)|CSM(HWKANA_7BIT)|CSM(JISX212)|CSM(GB2312)|CSM(KSC5601)|CSM(ISO8859_1)|CSM(ISO8859_7),
     CSM(ASCII)|CSM(JISX201)|CSM(JISX208)|CSM(HWKANA_7BIT)|CSM(JISX212)|CSM(GB2312)|CSM(KSC5601)|CSM(ISO8859_1)|CSM(ISO8859_7),
     CSM(ASCII)|CSM(JISX201)|CSM(JISX208)|CSM(HWKANA_7BIT)|CSM(JISX212)|CSM(GB2312)|CSM(KSC5601)|CSM(ISO8859_1)|CSM(ISO8859_7)
+#else
+    CSM(ASCII)|CSM(JISX201)|CSM(JISX208)|CSM(HWKANA_7BIT),
+    CSM(ASCII)|CSM(JISX201)|CSM(JISX208)|CSM(HWKANA_7BIT),
+    CSM(ASCII)|CSM(JISX201)|CSM(JISX208)|CSM(HWKANA_7BIT),
+    CSM(ASCII)|CSM(JISX201)|CSM(JISX208)|CSM(HWKANA_7BIT)
 #endif
 };
 
diff --git a/icu4c/source/common/unames.cpp b/icu4c/source/common/unames.cpp
index 71b2baff6c19..60b0de4f4e8d 100644
--- a/icu4c/source/common/unames.cpp
+++ b/icu4c/source/common/unames.cpp
@@ -999,6 +999,11 @@ enumAlgNames(AlgorithmicRange *range,
         uint16_t prefixLength, i, idx;
 
         char c;
+        
+        /* guard against invalid data */
+        if(count==0 || count>8) {
+            return true;
+        }
 
         /* name = prefix factorized-elements */
 
@@ -1025,7 +1030,7 @@ enumAlgNames(AlgorithmicRange *range,
         while(++start<limit) {
             /* increment the indexes in lexical order bound by the factors */
             i=count;
-            for (;;) {
+            while (i>0) {
                 idx = static_cast<uint16_t>(indexes[--i] + 1);
                 if(idx<factors[i]) {
                     /* skip one index and its element string */
@@ -1129,6 +1134,11 @@ findAlgName(AlgorithmicRange *range, UCharNameChoice nameChoice, const char *oth
 
         char c;
 
+        /* guard against invalid data */        
+        if(count==0 || count>8) {
+            return true;
+        }
+        
         /* name = prefix factorized-elements */
 
         /* compare prefix */
@@ -1154,7 +1164,7 @@ findAlgName(AlgorithmicRange *range, UCharNameChoice nameChoice, const char *oth
         while(++start<limit) {
             /* increment the indexes in lexical order bound by the factors */
             i=count;
-            for (;;) {
+            while (i>0) {
                 idx = static_cast<uint16_t>(indexes[--i] + 1);
                 if(idx<factors[i]) {
                     /* skip one index and its element string */
diff --git a/icu4c/source/common/unisetspan.cpp b/icu4c/source/common/unisetspan.cpp
index 209f0a86b541..c1cb152adfd8 100644
--- a/icu4c/source/common/unisetspan.cpp
+++ b/icu4c/source/common/unisetspan.cpp
@@ -93,6 +93,10 @@ class OffsetList {  // Only ever stack-allocated, does not need to inherit UMemo
     // If there is an offset equal to delta, it is removed.
     // delta=[1..maxLength]
     void shift(int32_t delta) {
+        // If OffsetList has zero capacity due to setMaxLength memory allocation error.
+        if (capacity == 0) {
+            return;
+        }
         int32_t i=start+delta;
         if(i>=capacity) {
             i-=capacity;
@@ -107,6 +111,10 @@ class OffsetList {  // Only ever stack-allocated, does not need to inherit UMemo
     // Add an offset. The list must not contain it yet.
     // offset=[1..maxLength]
     void addOffset(int32_t offset) {
+        // If OffsetList has zero capacity due to setMaxLength memory allocation error.
+        if (capacity == 0) {
+            return;
+        }
         int32_t i=start+offset;
         if(i>=capacity) {
             i-=capacity;
@@ -117,6 +125,10 @@ class OffsetList {  // Only ever stack-allocated, does not need to inherit UMemo
 
     // offset=[1..maxLength]
     UBool containsOffset(int32_t offset) const {
+        // If OffsetList has zero capacity due to setMaxLength memory allocation error.
+        if (capacity == 0) {
+            return false;
+        }
         int32_t i=start+offset;
         if(i>=capacity) {
             i-=capacity;
@@ -128,6 +140,10 @@ class OffsetList {  // Only ever stack-allocated, does not need to inherit UMemo
     // and reduce all other offsets by this minimum.
     // Returns [1..maxLength].
     int32_t popMinimum() {
+        // If OffsetList has zero capacity due to setMaxLength memory allocation error.
+        if (capacity == 0) {
+            return 0;
+        }
         // Look for the next offset in list[start+1..capacity-1].
         int32_t i=start, result;
         while(++i<capacity) {
diff --git a/icu4c/source/common/unormcmp.cpp b/icu4c/source/common/unormcmp.cpp
index fa8b85cea047..25555798ec3d 100644
--- a/icu4c/source/common/unormcmp.cpp
+++ b/icu4c/source/common/unormcmp.cpp
@@ -329,6 +329,9 @@ unorm_cmpEquivFold(const char16_t *s1, int32_t length1,
                      * the decomposition would replace the entire code point
                      */
                     --s2;
+                    if (s2==fold2) {
+                        return c1-c2;
+                    }
                     c2=*(s2-1);
                 }
             }
diff --git a/icu4c/source/common/ustrcase.cpp b/icu4c/source/common/ustrcase.cpp
index 58b5c5ef6235..ee8360db220e 100644
--- a/icu4c/source/common/ustrcase.cpp
+++ b/icu4c/source/common/ustrcase.cpp
@@ -1714,6 +1714,10 @@ static int32_t _cmpFold(
                      * the decomposition would replace the entire code point
                      */
                     --s2;
+                    if (s2==fold2) {
+                        cmpRes=c1-c2;
+                        break;
+                    }
                     --m2;
                     c2=*(s2-1);
                 }
diff --git a/icu4c/source/i18n/calendar.cpp b/icu4c/source/i18n/calendar.cpp
index 314ae506bec0..d130514c3a2f 100644
--- a/icu4c/source/i18n/calendar.cpp
+++ b/icu4c/source/i18n/calendar.cpp
@@ -3324,6 +3324,9 @@ int32_t Calendar::computeZoneOffset(double millis, double millisInDay, UErrorCod
             // 6 hour window would be sufficient for this purpose.
             int32_t tmpRaw, tmpDst;
             tz.getOffset(tgmt - 6*60*60*1000, false, tmpRaw, tmpDst, ec);
+            if (U_FAILURE(ec)) {
+                return 0;
+            }
             int32_t offsetDelta = (rawOffset + dstOffset) - (tmpRaw + tmpDst);
 
             U_ASSERT(offsetDelta < -6*60*60*1000);
diff --git a/icu4c/source/i18n/collationweights.cpp b/icu4c/source/i18n/collationweights.cpp
index 6a4c32bc6015..2029e8890d01 100644
--- a/icu4c/source/i18n/collationweights.cpp
+++ b/icu4c/source/i18n/collationweights.cpp
@@ -82,6 +82,9 @@ setWeightByte(uint32_t weight, int32_t idx, uint32_t byte) {
 
 static inline uint32_t
 truncateWeight(uint32_t weight, int32_t length) {
+    if (length == 0) {
+        return 0;
+    }
     return static_cast<uint32_t>(weight & (0xffffffff << (8 * (4 - length))));
 }
 
diff --git a/icu4c/source/i18n/nfrs.cpp b/icu4c/source/i18n/nfrs.cpp
index b063f7b6d816..9c75da3c31e5 100644
--- a/icu4c/source/i18n/nfrs.cpp
+++ b/icu4c/source/i18n/nfrs.cpp
@@ -604,8 +604,14 @@ NFRuleSet::findFractionRuleSetRule(double number) const
     int64_t numerator;
     {
         for (uint32_t i = 1; i < rules.size(); ++i) {
+            if (leastCommonMultiple == 0) {
+                return nullptr;
+            }
             leastCommonMultiple = util_lcm(leastCommonMultiple, rules[i]->getBaseValue());
         }
+        if (leastCommonMultiple == 0) {
+            return nullptr;
+        }
         numerator = util64_fromDouble(number * static_cast<double>(leastCommonMultiple) + 0.5);
     }
     // for each rule, do the following...
diff --git a/icu4c/source/i18n/rbt_pars.cpp b/icu4c/source/i18n/rbt_pars.cpp
index e2dbb058d5c5..36ec25dc8be7 100644
--- a/icu4c/source/i18n/rbt_pars.cpp
+++ b/icu4c/source/i18n/rbt_pars.cpp
@@ -1496,6 +1496,13 @@ int32_t TransliteratorParser::syntaxError(UErrorCode parseErrorCode,
                                           int32_t pos,
                                           UErrorCode& status)
 {
+    if (pos > rule.length()) {
+        pos = rule.length();
+    }
+    if (pos < 0) {
+        pos = 0;
+    }
+
     parseError.offset = pos;
     parseError.line = 0 ; /* we are not using line numbers */
     
diff --git a/icu4c/source/i18n/stsearch.cpp b/icu4c/source/i18n/stsearch.cpp
index c41e8de21dc1..e7e4f32dcbe2 100644
--- a/icu4c/source/i18n/stsearch.cpp
+++ b/icu4c/source/i18n/stsearch.cpp
@@ -290,6 +290,9 @@ StringSearch * StringSearch::safeClone() const
                                             getCollator(),
                                             m_breakiterator_,
                                             status);
+    if (U_FAILURE(status)) {
+        return nullptr;
+    }
     /* test for nullptr */
     if (result == nullptr) {
         status = U_MEMORY_ALLOCATION_ERROR;
diff --git a/icu4c/source/i18n/tridpars.cpp b/icu4c/source/i18n/tridpars.cpp
index 5c769dc39fbc..31eff2af0dce 100644
--- a/icu4c/source/i18n/tridpars.cpp
+++ b/icu4c/source/i18n/tridpars.cpp
@@ -907,6 +907,9 @@ void U_CALLCONV TransliteratorIDParser::init(UErrorCode &status) {
     ucln_i18n_registerCleanup(UCLN_I18N_TRANSLITERATOR, utrans_transliterator_cleanup);
 
     SPECIAL_INVERSES = new Hashtable(true, status);
+    if (U_FAILURE(status)) {
+        return;
+    }
     if (SPECIAL_INVERSES == nullptr) {
     	status = U_MEMORY_ALLOCATION_ERROR;
     	return;