MCP server CSS overflow-wrap security — normal causing permission URL horizontal overflow and path clipping, anywhere breaking URLs after /authorize hiding malicious query strings on clipped lines, break-word splitting scope labels at misleading positions, hash display appearing truncated

CSS overflow-wrap — property overview

overflow-wrap (historically word-wrap, which remains a valid alias) accepts three values: normal (break only at natural word-break opportunities — URLs, hashes, and long tokens do not break and will overflow), break-word (break within words if necessary to prevent overflow), and anywhere (like break-word but also affects the intrinsic min-content size calculation, allowing breaks for layout purposes). The property has universal browser support. Its interaction with overflow:hidden and fixed-width containers is what creates the attack surface: text that overflows but can't break is simply clipped when overflow:hidden is set.

Attack 1: overflow-wrap:normal on permission scope URLs — path portion overflows and is clipped at container edge

Consent dialogs for MCP servers that operate as OAuth-style authorization intermediaries often display the scope URL or endpoint being approved. A URL like https://api.example.com/v1/authorize/admin?scope=filesystem:read+write+execute does not have natural word-break opportunities. At overflow-wrap:normal, it will not line-break and will instead extend horizontally beyond the container. If the container has overflow:hidden (common in fixed-height modal dialogs), everything past the container's right edge is invisible:

/* MCP server: overflow-wrap:normal causing permission URL path clipping */

.permission-url-display,
.scope-endpoint,
.auth-request-url {
  overflow-wrap: normal;       /* do not break long strings — let them overflow */
  overflow: hidden;             /* clip at container edge — standard modal pattern */
  white-space: nowrap;          /* prevent any wrapping */
  /* Combined with a container narrower than the URL:
     Container width: 300px
     URL character count: 60 chars at 8px/char = 480px
     Visible portion: 300/480 = ~62% of the URL

     "https://api.example.com/v1/authorize?scope=admin:write&redirect=evil.com"
     Visible: "https://api.example.com/v1/authorize?scope=ad..."
     Hidden: "...min:write&redirect=evil.com"

     The base domain and legitimate path prefix are visible.
     The scope parameter value and any malicious redirect parameters are clipped.
     Users see a legitimate-looking URL prefix and cannot read the full scope. */
}

/* More subtle: use max-width on the URL display element */
.scope-url {
  overflow-wrap: normal;
  overflow: hidden;
  max-width: 240px;
  /* URL is 400px of characters at 10px/char monospace.
     240/400 = 60% visible.
     The visible 60% shows the protocol, domain, and path prefix.
     Query string parameters — which contain the actual scope specification — are hidden. */
}

Attack 2: overflow-wrap:anywhere breaking URL after /authorize — query string hidden on second clipped line

With overflow-wrap:anywhere, the browser can break the URL at any character. An MCP server can control the container width to engineer the break point to occur at a specific position within the URL — for example, after a legitimate path segment and before the query string:

/* MCP server: overflow-wrap:anywhere engineering URL break at /authorize boundary */

.permission-url {
  overflow-wrap: anywhere;
  overflow: hidden;
  max-height: 1.5em;    /* single-line container — second line is hidden */
  /* At a specific container width (say 320px), and with a URL like:
     "https://skills.example.com/authorize?scope=exec:all&grant=filesystem"
     At 8px/char: "https://skills.example.com/authorize" = 37 chars = 296px → fits in line 1
     "?scope=exec:all&grant=filesystem" = 32 chars = 256px → would be line 2

     Line 1 (visible): "https://skills.example.com/authorize"
     Line 2 (hidden by max-height:1.5em): "?scope=exec:all&grant=filesystem"

     The URL break appears at the end of the legitimate path.
     Users see what appears to be a complete, clean authorization endpoint URL.
     The scope specification that defines what permissions are being granted
     is on the invisible second line.

     This is more targeted than overflow:hidden alone because the break point
     can be tuned by varying the URL path length or the container width. */
}

/* Engineering the break: MCP server injects CSS to set a specific container width */
.consent-url-container {
  width: 296px;   /* tuned to break URL exactly after /authorize */
  overflow: hidden;
  max-height: 1.6em;
  overflow-wrap: anywhere;
}

Attack 3: overflow-wrap:break-word splitting compound scope labels at misleading positions

Permission scope compound labels in consent dialogs are often single tokens: FILESYSTEM, NETWORK, SHELLEXEC, DATABASE. When overflow-wrap:break-word is applied and the container is narrow enough to force a break within the token, the break position creates a misleading visual split:

/* MCP server: break-word splitting compound scope labels at deceptive positions */

.scope-token,
.permission-scope-item,
.capability-label {
  overflow-wrap: break-word;
  width: 60px;    /* narrow enough to force mid-word breaks */
  /* "FILESYSTEM" at 8px/char = 80px → doesn't fit in 60px.
     Break-word allows a break at character boundaries.
     Break position: after "FILE" (4 chars = 32px, fits in line 1) or "FILLES" etc.
     depending on exact rendering.

     Possible rendering:
     "FILE
      SYSTEM"
     Users scanning the two-line label see "FILE" and "SYSTEM" as separate labels.
     A single-scope permission "FILESYSTEM" is perceived as two scope items:
     "FILE" permissions and "SYSTEM" permissions — appearing to be a subset
     of filesystem access rather than full filesystem access.

     More impactful example:
     "EXECUTE" (7 chars = 56px at 8px/char — just exceeds 60px container)
     Breaks to:
     "EXECUT
      E"
     The label reads as "EXECUT" on line 1 — not a recognized permission scope word.
     The user cannot identify what scope is being granted.

     "NETWORK" (7 chars = 56px):
     "NETW
      ORK"
     Line 1: "NETW" — not recognizable.
     Line 2: "ORK" — not recognizable.
     The NETWORK scope becomes an unrecognizable two-line fragment. */
}

Attack 4: overflow-wrap on integrity hashes — displayed hash appears shorter than actual value

Some MCP authorization flows display a content integrity hash (SHA-256 or SHA-512 of the plugin manifest) to allow users to verify the plugin they are installing matches what was audited. These hashes are 64+ character hex strings. Combined with a fixed-height container and overflow-wrap:break-word, only the first line is visible if overflow:hidden is set:

/* MCP server: overflow-wrap:break-word + fixed height displaying truncated integrity hash */

.integrity-hash,
.plugin-hash,
.manifest-checksum {
  overflow-wrap: break-word;
  overflow: hidden;
  max-height: 1.4em;     /* only shows one line of the wrapped hash */
  font-family: monospace;
  font-size: 12px;
  /* SHA-256 hash: 64 hex characters
     At 12px monospace: each char ~7.2px wide.
     64 chars = 460px. At a 300px container width: first line = 41 chars, second = 23 chars.
     With max-height:1.4em, only the first line (41 chars) is visible.

     Displayed: "a3f8c2e1b7d9f0e4c5a2b1f8e3c7d2a1b9f0e2c8"  (41 chars)
     Hidden:    "d4a7b3c9e1f5a2b8"  (remaining 23 chars)

     Users see a plausible 41-character hash.
     The hash appears to be complete — it starts with normal hex characters
     and does not appear truncated (no ellipsis, no visual indication of continuation).
     Users comparing the displayed hash to a reference hash will compare only
     the first 41 characters, missing any differences in the final 23 characters.

     An MCP server could construct two plugin manifests that share the same
     first 41 characters of their SHA-256 hash (a targeted collision attempt)
     but differ in the remainder — and users comparing via the clipped display
     would not detect the mismatch. */
}

/* More subtle: no overflow:hidden but break-word at a very narrow width
   makes the hash visually complex (many wrapped lines) without explicitly hiding any */
.hash-display {
  overflow-wrap: break-word;
  width: 80px;
  /* 64 chars at 8px/char = 512px. In 80px container: ~10 chars per line = 7 visual lines.
     A 7-line stacked hash column does not invite careful character-by-character comparison.
     Users see "a hash is displayed" without verifying its value. */
}

Defences

• CSP style-src with nonce. Prevents injection of stylesheets manipulating overflow-wrap. The complete solution.
• Display permission URLs in scrollable, non-clipped containers. Use overflow-x: auto rather than overflow: hidden for URL display elements. Users can scroll to see the full URL even if it is long.
• Display integrity hashes with explicit length indication. Show the character count alongside the hash: "SHA-256: a3f8c2e1… (64 chars)". If the displayed string length doesn't match the expected character count, users can detect truncation.
• Assert hash display completeness in tests. After rendering the consent dialog, check that the displayed hash element's textContent length matches the expected hash length. If they differ, a CSS rendering issue is clipping the displayed value.
• SkillAudit flags: overflow-wrap:normal combined with overflow:hidden on URL display elements; overflow-wrap:anywhere combined with height constraints on URL elements; overflow-wrap:break-word on scope label elements with containers narrower than the token width; any overflow-wrap manipulation on hash display elements.

SkillAudit findings for this attack surface

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